Conventional greenhouses have long been constructed with transparent or translucent outer walls to admit the sun's rays during the daytime and utilize solar energy to promote the growth of the plants within the greenhouse. However, such outer walls allow a high degree of heat loss to the outside cooler air during the nighttime, particularly in winter when the heat differential is greatest, requiring a constant circulation of warm air to maintain the desired inner temperature.
When one considers that there are approximately 10,000 acres of greenhouses in the United States, the cost of fuel such as natural gas or oil necessary to supply heat to maintain the desired temperature in greenhouses during the nighttime, particularly in low temperature areas, is excessive, especially in view of the current shortages of such fuels.
Various devices have been proposed to reduce the heat losses in greenhouses, including curtains and shades, folding doors and special glazing methods. All of these proposals have obvious disadvantages, including maintenance expense.
Greenhouses having walls and roofs of plastic sheeting are in substantial use and the sun's rays are freely transmitted therethrough during the daytime, but in cool or freezing weather large amounts of warm air must be circulated within the enclosure in the nighttime. It has been proposed to provide an air space between two transparent walls for thermal insulation. This reduces the heat loss somewhat but not enough to effect a substantial reduction in the fuel required to supply additional heat in the nighttime in cold weather localities.
U.S. Pat. No. 4,021,972 discloses an air supported structure having an outer wall formed of an outer plastic imperforate wall and an inner plastic perforate wall with the space between the walls filled with thermal insulating material such as polystyrene beads. The walls are connected along parallel lines forming sectional chambers and the beads are blown separately into each chamber, the filled chambers forming a permanent insulated wall supported by differential air pressure maintained within the enclosed space. Obviously, this structure is not designed nor adapted to function as a greenhouse for several reasons, including the fact that the insulated wall would not transmit the sun's rays.
U.S. Pat. No. 4,108,373 discloses a method of day and night air conditioning a greenhouse having spaced inner and outer glass walls by circulating a heat transporting liquid between the walls containing an optical filter during the daytime and storing the absorbed heat in a reservoir, and then circulating the liquid during the nighttime to use the absorbed heat to maintain the desired temperature within the enclosure. Obviously, this process would require large heat storage capacity and would be expensive to operate.
U.S. Pat. No. 3,903,665 discloses a window panel for use in a building structure, said panel having rigid walls of glass or the like which are maintained in spaced-apart relation to form a chamber of fixed volume, and means for moving a granular insulating media into and out of the chamber for reducing heat loss through the panel during the nighttime and increasing energy transmission during the daytime. The operability of this system is dependent upon using rigid or semirigid window panels of relatively small size so that the insulating material can be blown into and evacuated from the chamber without serious coagulation causing obstructions at the inlet and discharge conduits.